Updated July 7, 2017
All three components of the fire triangle – fuel, heat and oxygen – must be present to have fire. A fire will burn until one or more of the three components are removed – the basis of traditional fire extinguishing methods.
Firefighters are trained to identify the signs that a fire will lead to an arc flash (flashover) or backdraft.
If gases (the fuel) rise to the ceiling and begin to spread, and nothing is done to ventilate or cool the room, a flashover – the sudden, simultaneous ignition of everything in an area – can occur. Due to intense temperatures achieved of as high as 1,000 degrees Fahrenheit, even a firefighter in full protective gear is unlikely to survive a fire arc.
Signs a flashover are imminent include dense black smoke with tightly packed curls, dense black smoke that pushes out of an opening; and smoke that has accumulated as low as a doorknob, with fire visible below.
In a backdraft, a fire burning in a confined area consumes all the oxygen, and flames are no longer visible. As temperatures increase, the gases expand and pressure builds. Any opening that allows oxygen to enter the space will result in the explosion of the pressurized gases.
Firefighter perspective on backdraft
A question posted recently on Quora asked, “How do you deal with backdraft when fighting a fire?” Firefighter Ben Urwin gave his opinion on the topic below. Check it out and add your own thoughts in the comments.
Backdrafts or smoke explosions can occur in fires that are severely ventilation-limited. This means that the fire’s aggravation is limited by the amount of oxygen available to it. The other two necessary ingredients for backdraft are available in abundance: heat and fuel. In this instance, the fuel is the smoke being released by the fire which contains a large amount of carbon monoxide – a flammable gas.
When these conditions are encountered, the goal is to remove as much heat and smoke from the structure as possible before introducing any new oxygen. Most of the time, this is accomplished by cutting an opening in the roof and refraining from opening any windows or doors on or below the fire floor until conditions have changed.
This means that no search and rescue can occur and no water is being put on the fire until the vertical ventilation can be completed. This can be difficult to understand for people watching the operations, especially if the house happens to be theirs!
However, in a fire where the conditions are right for a backdraft to occur, there is absolutely no chance of finding anyone alive in the building. Fire growth is fairly slow and it may even be auto-extinguishing due to lack of oxygen. The biggest danger, by far, is that of a backdraft and so the risk must be eliminated before the firefighters can move on to other priorities.
